This may be of interest:
Being in a relationship, for me, has always caused me to become depressed. Two teenagers with mental illness together is
not healthy. Going in and out of the same relationship and being unable to cope without said partner is
not healthy.
At first, yes, breaking up was awful for me, but now I am completely stress-free. I have realized that I feel much better mentally, without a partner.
I do not advise that you force or hint for them to break up, but I am worried that your son is only hurting his girlfriend. I was in a relationship with someone that had depression, and I felt that it was my job to take care of his mental health. Whenever I went though mood swings, and he came to me looking for support, I was unable to give that support because I also needed it.
I personally went though a time in which I felt the same as your son. My school paid for a counselor, and after healing over time, and having someone to talk to, I am feeling much better mentally. If you let him go and deal with this himself, I am afraid that he will think that you are giving up on him. I know that he is 18, but please stay by his side and be supportive. I really mean it, because I don't know where I would be without my parents' support.
Find a counselor that he can talk to. Someone that is younger may be better. I found it a bit awkward to talk to someone that was much older than me- just because I felt as if they "wouldn't understand me" (Yep, all teenagers say this). Medication will also be great, although some side-effects of antidepressants cause depression for some reason. I personally took medication that was a sort of "multi-use" medication? It worked for depression, anxiety, and I think ADD? I took it for 7 years, and I am proud to say that last month, I finally got the okay to stop taking it! The right medication and therapy will do wonders of help, even if it takes a lot of time